I wait at least an hour after a meal and prefer two, but that's not always practical. Each body has its own preferences, so be prepared for some trial and error before you figure it out.
I'd never let having recently eaten deter me from working out but it might determine what kind of workout and how vigorous it is. I don't think that a brisk walk, even immediately after a meal, would ever be a problem. Literature is full of references to characters taking an after luncheon 'constitutional', after all.